All woods have a fibro-vascular tissue composed of cellulose and its components belonging to the subdivision called spermatophytes (IV) in the plant kingdom (with the single exception of tree ferns). The spermatophytes can be further subdivided into two classifications; gymnosperms or "softwoods" and the angiosperms or "hardwoods". It must be emphasized that the terms softwood and hardwood have no bearing on the density or degree of hardness of such woods but moreover refers to their classification. Some woods that are classified as softwoods, such as yellow pine, are physically harder than some woods that are classified as hardwoods such as aspen or basswood. Further, angiosperms can be again divided into very distinct classes: the monocotyledons or the palms, bamboos, canes and grasses and the dicotyledons (the majority of angiosperms that provides us with useful woods).
Since a living tree contains very large amounts of water, lumbermen often refer (at various stages from the initial cutting of a tree up through the sawing and drying of lumber) to the moisture content (MC) of the wood. The moisture content of the wood, usually expressed in a percentage, is a ratio of the amount of water in a piece of wood that is compared to the weight of such wood when all of the moisture has been removed. One of the methods that is employed (the "moisture content on the oven-dry basis") to determine the MC of wood at any stage during the lumber production process is to weigh a given sample of wood and record such weight (the "wet weight"). The sample is then placed into an oven and heated (at temperatures not to exceed 217 F.) until all of the moisture has been removed (the "oven-dry weight") and that weight is recorded. It can be determined that the oven-dry weight has been reached when, after weighing at various intervals, the sample stops losing weight. The oven-dry weight is then subtracted from the wet weight and the resultant is then divided by the oven-dry weight. That resultant figure is then multiplied by 100 to determine the percentage of MC. The formula can be represented as follows: ##EQU1## The type of units employed for the above calculation, i.e. ounces, grams, pounds, kilograms, etc., is not important as long as all weights are recorded in the same type of units since the calculations are based upon a ratio of such weights. Other methods of determining MC have been developed as well as electronic machines that compute the MC based upon known electrical and other reactions. Regardless of the method employed to determine such MC, a working knowledge of moisture content and how it effects wood is important to the present process.
When a hardwood tree such as red or white oak, ash, maple, poplar, or any one of the many species of hardwoods that are used in the production of wood products is initially cut down, it has a MC of anywhere from about 60% to 100% (this moisture content has been found to be even higher, as much as about 150% for some softwoods). This is called the "green moisture content". This moisture or water has to be removed or dried from the wood in order to make the wood usable in any phases of the hardwood lumber industry that require either air dried and/or kiln dried lumber. The drying or curing of hardwoods thus comprises the controlled removal of water from the wood to a level (approx. 6% to 8% MC) where the wood becomes sufficiently stable for fabrication into various products. However, the present green wood treating process is performed prior to the drying or curing process. Although in some instances the cooling of the wood after heating for a predetermined time may be confused with a kiln drying schedule, the "curing" process or "curing" as used herein refers to moisture removal by the controlled act of air drying, kiln drying, or a combination of both.
After a tree is felled and is sawn into lumber of various sizes and types, it is stacked in a particular manner in preparation for the drying and/or pre-drying process. During this pre-drying or drying period, many problems may occur that can either damage, destroy or degrade the quality of the wood and render it less desirable and in some cases, not usable. The sawn lumber can develop cracks in the ends ("end checks"), cracks in internal portions of the lumber ("honeycomb" or "honeycombing"), cracks in the surface ("surface checking"), as well as many types of warps and bends ("cup", "bow", "crook", etc.). In addition to these problems, the sawn lumber can develop several types of stains and discolorations. During the history of the development of the technical aspects of lumber drying, most of the above mentioned problems have been handled somewhat by controlled drying and the application of chemicals during the drying process except for two very troublesome classes of stains, sap or blue stain caused by a fungus and chemical stains caused by the action of enzymes that are contained in the wood. These stains normally occur between the time that the tree is felled and prior to the beginning of, and during, the drying processes.
Blue stain is a fungal stain that occurs in the sapwood of the tree. Although the tree has several very distinctive layers that make up its cross-section, there are two broad areas known as "sapwood" and "heartwood". The sapwood comprises the living layers (parenchyma cells), growing layers (cambium layer) and semi-dormant cells which occur in the life processes of the tree that surround the central portion or heartwood of the tree. The heartwood contains stabilized cells that are hardened and laden with tannin, natural chemicals and resins. The stability of the cells in the heartwood and the presence of tannin, as well as the lack of the sugars and starches, minimize any intrusion of discolorations due to the blue stain and the chemicals stains in such heartwood cells.
Blue stain is currently believed to be caused by fungal activity which is promoted by four main elements. Those elements are: a) temperature above 50 F. (a reason that blue stain is more troublesome in the southern United States); b) presence of oxygen; c) presence of moisture; and d) presence of sugar and starch occurring naturally in living cells of the sapwood (parenchyma cells). It is commonly held that the elimination of one of these elements will control blue stain although the control of these elements is very difficult and therefore, rather ineffective.
Chemical stains such as sticker stain, sticker shadow and interior graying also only occur in the sapwood and are caused by the oxidation of enzymes that are present in the parenchyma cells of the sapwood fibers. Effective measures to control the chemical stains include controlling the of oxygen to the sawn lumber and completing the drying cycle of the wood as quickly as possible. However, a precise oxidation of the sawn lumber is difficult to achieve and control.
For blue stain or fungal stain, there are various chemical cures for this problem but these methods generally have been found to be expensive and harmful to the environment. Blue stain and chemical stain result in a major expense to the lumber industry. The present invention is directed to the treating of green wood prior to curing.
It is an object of the present invention to provide a process for treating green wood which minimizes or eliminates blue stain and chemical stains in the wood.